Add water marks or something , developers can just take your graphics off your website easily.

Thanks for the tip, but if some one does that I'll post an cracked version of their game on my site

But to be serious, I don't think watermarking is worth it. The risk for something like that happening is minimal.
Watermarked graphics are never as good. And would end up costing me customers.
I got the source files to my graphics, so it would easy to prove ownership unless some one forges an paypal transaction as prof of purchase. And if you could hack PayPal, I don't think you would spend you time stealing portfolio images. Not sure how Apple handles situations like that, so would be an opportunity to find out.

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